Actually, my day job employer sells (among other things) tennis apparel and equipment, and I registered it with the intent of transferring it to them and just getting reimbursed for the reg fee when we were talking about starting up an "experts" type company blog... but that never came to fruition.
I'm keeping it because it feels like it has some potential. I actually like the .net extension (though that wouldn't have stopped me getting the .com had it been available), given the connotations with tennis.
When I was originally looking, I found a few tennis blogs/forums/etc., but nothing with that great of a presence.